Endotoxin and thrombin elevate rodent endothelial cell protein C receptor mRNA levels and increase receptor shedding in vivo

The endothelial cell protein C receptor (EPCR) facilitates protein C activa tion by the thrombin-thrombomodulin complex. Protein C activation has been shown to be critical to the host defense against septic shock. In cell cult ure, tumor necrosis factor-alpha (TNF-alpha) down-regulates EPCR expression , raising the possibility that EPCR might be down-regulated in septic shock , We examined EPCR mRNA and soluble EPCR levels in mice and rats challenged with lethal dose 95 levels of endotoxin, Toxic doses of TNF-alpha failed t o alter EPCR mRNA levels in mice. Rather than EPCR mRNA levels falling in r esponse to endotoxin, as predicted from cell-culture experiments, they rose approximately 3-fold 6 hours after exposure to endotoxin before returning toward baseline levels at 24 hours after exposure. Soluble EPCR levels rose approximately 4-fold. Infusion of hirudin, a specific thrombin inhibitor, before endotoxin exposure almost completely blocked the increase in EPCR mR NA and soluble EPCR, Consistent with the idea that the responses were media ted by thrombin, thrombin infusion (5 U/kg of body weight for 3 hours) resu lted in an approximately P-fold increase in EPCR mRNA and soluble EPCR. Inc ubation of rat endothelial cells with thrombin or murine protease-activated receptor 1 agonist peptide resulted in a a-fold increase in EPCR mRNA, The se results indicate that thrombin plays a major role in up-regulating EPCR mRNA and shedding in vivo, (C) 2000 by The American Society of Hematology.